Depends where the steam is coming out of. If it's coming out of the cap, then it could be something as simple as your cap not being tightened enough. Yet, if it's leaking out of the sides, bottom, or anywhere else then it could be a radiator leak, which will cause your car to overheat at times, and could lead to other mechanical malfunctions, therefore check to see where it's at and if it's a radiator leak, i reccomend getting it fixed sooner than later.